The claimed invention relates to rotors of motorcycle brake discs.
It is known that, when braking a motorcycle, a main part of the load (up to 75% and even more) is applied to the front wheel, and, consequently, the front wheel brake is loaded much more than the rear wheel brake. Therefore, the front brake disc is heated significantly greater than the rear brake disc during braking, and it is the reason that explains difference in structure of front and rear brake discs of modern motorcycles.
Front brake discs have a greater external diameter (300-330 mm) and consist, as a rule, of a rotor having a width about 35 mm and a thickness from 4 to 7 mm (usually, 4.5-6 mm), to which surface a braking force from brake pads is applied directly, and an aluminum carrier (“spider”) that is used for fixation of a brake disc on the front wheel. As the material of the motorcycle brake disc rotor to which the requirements of a stable and rather high (as a rule, from 0.3 and higher) friction coefficient with the pads in a broad temperature range, minimum wear, high rigidity and corrosion resistance are applied, a stainless steel, usually of martensite class (of 20X13 type in Russia or Steel 420 according to AISI, USA) is mostly used. The aluminum carrier (“spider”) of the front brake disc is connected to the rotor either by a rigid tie, and then the whole brake disc is a rigid structure, or so called “floating” discs are used, and then the hub is connected to the rotor through special bushings. In the latter case a steel rotor, which is heated significantly during braking (up to 500° C. or more) and expands due to heating, practically does not deform itself and does not deform the aluminum carrier, and the flat form of the rotor is maintained due to moving in a “floating contact”, which is principally important during braking.
The use of such a composite structure for the motorcycle front brake disc enables not only to decrease its weight, but also reduce heating of the carrier of a brake disc (and the wheel itself), since heat transfer from the rotor heated during braking through floating contacts is significantly poorer.
The rotor of the motorcycle rear brake disc experiences significantly lower heat load. As a result, rear brake discs are of a smaller size (external diameter, as a rule, is not more than 220-230 mm) and represent, actually, a simple steel rotor with a thickness ranging from 4 to 7 mm (preferably about 5 mm) on which mounting holes are arranged for fixing the rotor on the rear wheel.
Many producers manufacture motorcycle front and rear brake discs comprising, as the main working element, a brake disc rotor. On the whole, except for several parts of their designs, motorcycle front and rear wheel rotors correspond to the above description. In particular, FIG. 1 shows examples of brake discs and rotors for motorcycle front and rear wheels, which are produced by Spacek Product company (Czech Republic, website: wwwdotgoldfrendotcom). Using of through slots (holes) should be noted that are made in the axial direction (transversely to the rotor friction surfaces), and/or transverse grooves on the rotor friction surface that are made, as a rule, in the radial direction and are used for removing dust and pad erosion products. The edges of such holes (grooves) are dust removers and act so as to cut pressed pad dust and discard it to the outside. A specific form of slots (holes) or grooves may differ and is determined by the requirements of sufficient strength and rigidity of the rotor as well as by aspects of visual attractiveness of the rotor design.
Thus, the known rotors of the motorcycle brake discs are an annular body having opposite annular friction surfaces as well as radially external and radially internal lateral surfaces. A distance between the flat friction surfaces (i.e., a rotor thickness) is 4-7 mm, preferably 5-6 mm, a distance between the radially external and radially internal lateral surfaces (i.e., a rotor width) usually corresponds to a width of a brake pad, is approximately 25-35 mm, and may be varied in the circumferential direction (see FIG. 1). As a rule, a plurality of through slots in the rotor axial direction and/or grooves in the radial direction is made in a rotor, but in any case a motorcycle brake disc rotor is a one-piece article made of one blank. In a majority of cases this blank is a sheet of heat-resistant stainless steel, and in some cases ceramic (composite) rotors are applied.
Conventional technical solutions enable to reduce a weight of a motorcycle brake disc rotor (and a brake disc on the whole) to a certain extent without the losses in braking performance, but the weight of rotors and their moment of inertia remain rather big, especially in view of the fact that rotors relate to motorcycle unsprung components which weight reduction is particularly important. A weight and a moment of inertia of a rotor have a significant effect on dynamics and maneuverability of a motorcycle (e.g., an angular velocity of wheel axis turn, which is also associated with motorcycle turn speed, depends on a wheel moment of inertia), and on fuel consumption, and, therefore, rotor weight reduction is very important.
A significant shortcoming of conventional rotors is their considerable heating in hard braking modes, especially during sporting events as well as in hot weather. It is for the purpose of preventing a braking fluid from overheating (since such overheating may cause brake failure) the rotor weight and dimensions are to be increased. An increase of the dimension, i.e., a surface area of a brake disc rotor, increases its cooling rate, but this causes a fast increase of a moment of inertia which for practically permanent thickness and width of a rotor is proportional to the third power of its radius.
An incident airflow cools conventional brake disc rotors in the laminar flow mode. Actually, the Reynolds number Re for air flowing around a rotor is determined by the expression Re=(V*L)/v, where V is a velocity of an incident airflow, L is a rotor ring width, v is air kinematic viscosity. Since L is app. 3 cm, v is app. 2.5*10−5 m2/s (in the temperature range from 50 to 350° C. kinematic viscosity raises from 1.*10−5 m2/s to 5.5*10−5 m2/s), then, even at the speed of 180 km/h (50 m/s) Re<6*104. But a laminar flow becomes a turbulent flow at Reynolds number Re>3*105 Yudin, B. N. “Technical Thermodynamics. Heat Transfer” (in Russian). M.: Higher School Publishers, 1988. 479 pages; p. 276).
Heat transfer in the laminar mode is comparatively inefficient. Transition to the turbulent mode of flowing around would enable to increase cooling speed significantly, but a ten-times increase in an incident airflow velocity or, practically, ten-times increase in rotor dimensions is impossible.
Slots and holes in conventional rotors, while improving operation of braking pads, only reduce an efficient area of heat exchange with the ambient air without significant turbulizing an airflow (which would improve a cooling mode).
And it is a rotor cooling speed that mainly determines rotor dimensions and weight, that is, if cooling conditions are better, then rotor dimensions may be decreased, and, correspondingly, a weight and, especially, a moment of inertia of a brake disc could be reduced too. Furthermore, at a significant lowering of a rotor maximum temperature during operation due to better cooling (e.g., to 300° C.) it may become possible to use light alloys for rotors (perhaps with corresponding coatings providing, first of all, a required friction coefficient and wear resistance for brake pads), which would contribute to a sharp decrease of a brake disc weight without significant complicating the technology of manufacturing a brake disc.
A significant improvement in brake disc cooling efficiency may be achieved, if a rotor has internal through cooling channels going from the radially external lateral surface to the radially internal lateral surface and serving for inside cooling the rotor annular body.
It is essentially important for the said high efficiency of heat exchange with an incident air in the internal through cooling channels of a rotor that such channels are made in a rather fast rotating part, since, if through channels of similar section and length (the relation between a channel length to minimum transverse size is 10 to 15) are immovable, no turbulization of an airflow takes place, and heat removal is inefficient ([1]; p. 294).
Actually, at an incident airflow speed V and a channel downstream length (i.e., in fact, a rotor width) L a characteristic time during which a given air portion is in a channel is T≈L/V. The time, during which a rotor, due to its rotation, will move by the channel length, is τ˜δ/V1 where: δ is a characteristic transverse size of a channel (its “width” in the circumferential direction, a channel may have a variable width); V1 is an average circumferential speed of a rotor. For a real rotor having an annular shape with a relatively small width in comparison to an average radius, which rotor rotates jointly with the wheel, the following relation is correct for V1: V1≈V·(Rr/Rt), where Rr and Rt are an average rotor radius and a motorcycle tire radius, respectively. The condition of collision of an airflow passing through a channel with the channel side walls has the obvious form τ<T, and, by using the above expressions, it is possible to find the condition for the parameters of rotor channels:δ<L(Rr/Rt)  (1)
For rotors of front brake disc of modern motorcycles of a rather high level the relation Rr/Rt is ˜0.45 (Rr is ˜155 mm, Rt is ˜330 mm), an average rotor width L is ˜30-35 mm, and it can be found from the relation (1) that δ<15 mm. For rotors of rear wheel brake discs the relation Rr/Rt is smaller and equals to ˜0.35, which at the same average rotor width gives the condition for a channel transverse size δ<10-11 mm.
At the optimal (also see below) channel transverse width of ˜5 mm the formulated condition (1) is complied with twofold reserve, and the possibility of colliding with the “vertical” channel walls only increase turbulization of a flow in the internal through cooling channels.
The geometric parameters of the internal through cooling channels are determined by the operating conditions of a motorcycle brake disc rotor, first of all by the requirement of sufficient local rigidity of the vertical walls of these channels (a thickness of the channel vertical wall is the thickness of a solid material layer “over the channels”, which adjoins the rotor friction surfaces that are acted on, i.e., pressed by brake pads), as well as by the requirements to the whole rotor rigidity at its preset dimensions (it is already said above that the diameter of a brake disc rotor for the motorcycle front wheel is ˜320 mm, the thickness is in the range of ˜5-6 mm) and by the expediency of decreasing a rotor weight. As a result, a range of possible parameters of the internal through cooling channels is limited irrespective of a specific variant of making them.
Thus, a deflection of the vertical walls of the internal through cooling channels under the pressure exerted by the brake pads is increased in direct proportion to the fourth power of the channel width and in inverse proportion to the third power of thickness of its vertical walls. As a result, a minimum thickness of the vertical walls of such channels is 1 mm, which determines the channel maximum height of 5 mm (for the integral rotor thickness of 7 mm), and an optimal channel height for the currently common rotors having a thickness app. 5 mm is 2-3 mm. Similarly, in order to ensure a rather high local rigidity of a rotor (rigidity in the area “over the internal through cooling channels”), a preferable width of these channels is in the range from 3 to 8 mm. In such a case a number of internal cooling channels in a rotor is 60-100.
The relation between the channel width and the channel wall width or, which is the same, the relation of a distance between the axes of neighboring internal through cooling channels to a characteristic width of a channel (since the channel shape may differ from the rectangular one) is determined, on one hand, by the condition that a surface area that actually ensures a friction force should be rather great. On the other hand, if a channel width is small in comparison to a distance between the axes of neighboring channels, then a volume of maximum efficient heat removal is reduced as well as a rotor weight is changed insignificantly. The joint action of these two opposite factors leads to the condition that the relation of a distance between the axes of the internal through cooling channels to their width does not exceed 4, preferably does not exceed 2.
A minimum width of the internal through cooling channels is, preferably, ˜1 mm and is determined by the two factors:                the technological factor: at a smaller channel width and an optimal relation of the width and a distance between the axes of such neighboring channels their number in a rotor becomes too great; thus, when the distance between the channel axes is 2 mm, their number in the brake disc rotor of the front wheel exceeds 400;        the operational factor: channels of small section are easily clogged with dust and contaminants, they are more difficult to be cleaned, when necessary.        
Similarly, a minimum channel height is determined by the operational reasons as well as by a reduction of heat removal efficiency in an internal through cooling channel (since a share of such channels in the rotor section becomes insignificant) and, practically, by maintenance of a rotor weight; the preferable height of the internal through cooling channels is at least 1 mm.
Thus, in order to ensure a significant weight and moment of inertia reduction for a ventilated rotor having internal through cooling channels with maintenance (or small change) of its full thickness, it is advisable to make ˜50-80 such channels with a characteristic width of ˜3-6 mm and a height of 2-3 mm in such a rotor. A greater height of the channels will inevitably lead to increase in the full thickness (height) of a rotor, which is, in principle, possible, but will require to alter (modernize) brake system supports currently used.
Various embodiments of a brake disc rotor with internal through cooling channels are proposed in the following patents ([2]: Fr 2927389; [3]: EP 1016803), Patent [2] relates to brake disc rotors of motorcycles and cars, and Patent [3] relates to brake disc rotors of bicycles.
Since it seems technically unreal to make such channels with a height not more than 2.5-3 mm (when the full thickness of a rotor is app. 5 mm) and a length app. 30 mm (corresponding to a rotor width) in one-piece article at a reasonable price of the finished article and subject to the fact that a number of channels in one rotor should by app. 60 or more, Patents [2], [3] propose to make brake disc rotor (or a whole brake disc) components manufactured separately.
Patent [2] proposes a brake disc with internal through cooling channels, where two annular friction surfaces are the external surfaces of two parallel steel plates kept at a distance from each other by spacers that are welded to the inner sides of these plates, which ensures rigid connection of the rotor elements to each other. In particular, such spacers may be rods of rectangular section that are arranged essentially radially (FIG. 15 in [2]), and this embodiment just forms internal through cooling channels going from the radially external lateral surface to the radially internal lateral surface of the rotor.
The proposed embodiment of the rotor provides a significant improvement in its cooling, but has extremely low manufacturability due to a great number of constituent elements, is difficult in assembling and practically does not ensure a reduction of the rotor weight. Indeed, in order to provide required rigidity of the rotor outer layers during braking with a small number of spacers, it is necessary to increase the thickness of the said outer layers significantly and, respectively, the rotor weight (that may even exceed the weight of a solid rotor), and with rather thin friction layers it is necessary to use a great number of spacing rods (˜70 or more rods with a length of ˜30 mm and a section of ˜4*2.5 mm), which is extremely non-practicable from the technological point.
Indeed, when making a rotor according to [2], it is necessary, at first, to arrange precisely all spacing rods (˜70 pieces!) on the inner side of one (the first) of two steel plates, to weld all the spacers to this plate, while ensuring that the metal will not be splashed or pressed out to the opposite sides of the spacers, to which the inner side of the second steel plate will then be welded. The presence of a splash of the metal or the pressed-out metal will cause (after welding of the second plate) mutual obliquity of the plates, the presence of local stress concentrators, etc. Obliquity may be corrected by grinding or milling the welded rotor as a one-piece article, but appeared concentrators may not be removed. Therefore, apart from other operations, control and additional treatment of the welded spacer free surfaces will be required after the first stage of welding, and, evidently, assembly and precise positioning of a great number of elements complicate the manufacturing of the rotor according to [2] also.
Patent [3] proposes a method for making a ventilated brake disc for a bicycle, which has internal through cooling channels, where a corrugated plate that is made by deforming in one operation is used as a spacer between the rotor outer flat layers, this corrugated plate has circumferentially alternating projections and recesses. All the three elements (layers) of this proposed rotor are made of stainless steel, and rigid connection of the rotor components is ensured due to using a nickel bonding paste in the areas of contact between the outer plates with a spacer and subsequent heating the whole assembly to a temperature above the austenitic transition temperature.
The side walls of the internal through cooling channels in this rotor are inclined regions of the corrugated intermediate layer that connect the projections and recesses, and the other two (vertical) channel walls (along the rotor thickness or axis) are, on one side, the inner (relative to the rotor) surface of the intermediate layer and, on the other side, the inner surface of the outer layer. That is, the thickness of the channel vertical wall on one side is equal to the thickness of the outer plate, and the thickness of the channel wall on the other side is equal to the sum of the outer layer thickness and the thickness of the plate which the corrugated intermediate layer is made of.
In this embodiment of a ventilated rotor the height of the internal through cooling channels is determined by the relationh=H−d,  (2)where: H is the full height of a corrugation, d is the thickness of a plate which a corrugated spacer is made of. For the rotor described in [3] H=3.2 mm, d=0.6 mm and the channel height is 2.6 mm, the side walls of the corrugated intermediate layer have approximately the same height (length). The proposed embodiment of the rotor simplifies its assembly significantly, but, in spite of improved cooling mode, it does not enable to reduce the weight of the whole rotor even for a bicycle (see below). In particular, the inventors of [3] describe a ventilated rotor which outer layers are formed by plates having the thickness of 0.8 mm, and the corrugated spacer is made of a plate with the thickness of 0.6 mm. Since the whole area of the corrugated spacer, as follows from description of its shape in [3], is significantly greater than the area of the outer layers, the weight of this rotor knowingly exceeds the weight of a similar solid rotor with the thickness of 2.2 mm. At the same time, a thickness of modern non-ventilated brake discs is usually 1.8-2.2 mm. Correspondingly, a greater rotor weight automatically ensures its lower heating during braking even without internal cooling channels.
Furthermore, the proposed ventilated brake disc rotor has significantly greater dimensions; thus, the full thickness of the rotor proposed in [3] is 4.8 mm, which is app. 2.4 times greater than the thickness of standard brake discs.
Further, it is to be taken into account that in the proposed embodiment of the ventilated rotor the rigidity (including local one) is practically fully determined by rigidity of the corrugated intermediate layer, first of all by the height of the corrugation itself and the thickness of its side walls. Correspondingly, if this known technical solution is used in a brake disc rotor for a motorcycle, the corrugated intermediate layer rigidity should be increased many times, since pressure exerted by brake pads on rotor friction surfaces is significantly greater during braking a motorcycle than a bicycle. Moreover, with growing pressure exerted by pads tangent stresses are also increased in points where the intermediate layer is connected to the outer layers (apart from stresses arising due to friction of pads on friction surfaces), which reduces the lifetime of the assembled construction.
Then, with increasing a thickness of the source plate which is used for forming the intermediate layer, the making of a structure corrugated circumferentially is sharply complicated. Calculations show that a thickness of inclined (lateral) parts of a corrugated structure in the brake disc rotor for a motorcycle should be at least ˜1.6 mm. In such a case, in order to obtain the height of 2 mm of the internal cooling channels, it is necessary to ensure that the full height of the corrugation is H>3.6 mm. The making of such a part is extremely complex technological task for a rather thick source plate, especially with regard to anisotropy of its plastic properties and different degree of plate deformation at different distances from its axis, and the making of a deformed corrugated structure for forming a rotor with channels of complex section becomes practically unreal. Subject to a thickness of the rotor outer layers>1.4 mm (in a case of a motorcycle), the full thickness of the rotor becomes>6 mm (>6.4 mm), which significantly exceeds the dimensions of modern non-ventilated rotors, and the weight of the known ventilated rotor is not less than the weight of quality standard rotors.
Furthermore, local heating of opposite regions of the known rotor, especially at the initial braking stage, differs significantly due to a multiple difference of heated metal thicknesses of a rotor at practically similar heat release on both friction surfaces, which causes additional thermal stresses and provokes geometric changes (distortions) of the ventilated rotor proposed in Patent EP 1016803.
Thus, the known ventilated brake rotors with internal through cooling channels are not manufacturable and do not ensure a weight reduction for a motorcycle brake disc rotor.